Capacitor to reduce switch off bang

[cretaceous]

hi

my cheap marshall guitar amp makes a really loud bang when I switch it off..

I've read that soldering a capacitor across the mains switch might reduce this noise
(presumably across live and neutral - UK mains 240v)

but what value and voltage and what capacitor type should I use?

 

[DF96]

Not across the switch, but across the mains transformer primary. Value 5-10nF. Voltage/type has to be one designed for safe use across mains: X or Y type.

 

[Printer2]

Which model? Doubt a cap on the mains will help.

 

[Enzo]

I'd be inclined to agree. Try turning your volume/gain controls to zero. ANy difference?

 

[cretaceous]

thanks for replies - it's an MG15 CDR
yes I turn volume to zero and turn reverb down but it still sounds like a gunshot
(overdrive is turned down too)

I've tried hitting the switch quickly and tried pushing it very gently.. no difference

ok i just realised it has a headphone jack so I could plug in a dummy jack but if I did that should I add some 8 ohm resistors across the output(s)?
(am not going to plug in my headphones, that bang will damage them)

 

[DF96]

A sound like a gunshot could well be caused by the switch-off transient in the mains circuit getting induced into a signal circuit. The other switch-off sound is a dull thud when the caps discharge, usually a few seconds after switching.

 

[pdul]

Try to put 10nf across the switch to kill the rf spike and spark in the switch, still x or y type.

 

[Printer2]

http://www.amparchives.com/Amp Arch... & Layouts/MG Schematics/Marshall MG15CDR.pdf

Can not see anything obvious. Try the chip amp guys.

 

[pdul]

I see great potential for soundshaping/ opamp rolling in that amp

 

[DF96]

The mains caps are on the wrong side of the switch for arc suppression. They will provide some crude filtering. Add a similar cap directly across the primary.

 

[cretaceous]

just got round to doing this - works perfectly - thanks !
(silent on switch off and a polite pop a second later)

 

[DF96]

Good. It will have to remain a mystery why the original designers put the caps in the wrong place.

 

[cretaceous]

aren't they smoothing caps for the psu? (I thought a pair of electroltyics always did that)

 

[DF96]

I means the caps across the mains supply. If they had put them on the transformer side of the switch rather than the mains side then they would have exactly the same filtering effect when switched on but in addition stop the switch from arcing when switching off. Also, they would not be exposed to continuous mains voltage when the item is plugged in but switched off so they would probably last longer. Many mains caps gradually reduce in value as mains voltage spikes cause local erosion of the metal film. This makes them fail-safe (they don't short and go bang) but eventually ineffective.

 

[bleachershane]

just got round to doing this - works perfectly - thanks !
(silent on switch off and a polite pop a second later)

I have no idea if you're still on this forum but I'm having exactly the same problem with two MG15CD amplifiers, one far worse than the other, like a gunshot, (I'm assuming that reverb aside the circuits are the same?) and was wondering what value of cap to use?

If anyone else want to chime in with the best capacitor value and type, that'd be great.

 

[JonSnell Electronic]

Fit a 100nF 250v AC, Class X capacitor across the primary winding connection of the mains transformer and a 10nF 250v AC Class Y capacitor across the switch contacts. This will stop arcing if the mains switch and switch off surges to the TDA2050 amplifier.

 

[kapitiaudio]

I would also replace the mains switch, sounds like it's arcing excessively.

 

[bleachershane]

Fit a 100nF 250v AC, Class X capacitor across the primary winding connection of the mains transformer and a 10nF 250v AC Class Y capacitor across the switch contacts. This will stop arcing if the mains switch and switch off surges to the TDA2050 amplifier.

Thanks for that, can get both of those locally. Fingers crossed it lessens if not removes the problem.

 

[bleachershane]

I would also replace the mains switch, sounds like it's arcing excessively.

Another good tip, will look into getting a Marshall power switch, are they pretty standard across most models?

 

[DF96]

Alternatively, just put a 10nF X cap across the transformer primary. Replace it every 10-15 years.

A cap across the switch will need replacing every 5 years or so, but no cap here needed if there is one across the primary.